Measurements in which image data sets of the examination object are recorded with different X-ray energies, are generally referred to as what are termed dual energy procedures. Since the attenuation of the X-ray beams in the different materials is a function of energy, it is possible to use this measurement procedure to obtain additional information compared with a measurement using just one X-ray energy. This information can be used to differentiate more easily between different materials in the beam path between the X-ray source and the X-ray detector.
A typical example of the application of a dual energy procedure is CT angiography (CT=X-ray computed tomography). For such an examination a contrast agent is administered to the patient shortly before the start of the CT measurement, so that at the time of the recording the vessels or vessel lumina to be examined contain contrast agent. A contrast agent containing iodine is generally used, which because of its high atomic number results in significant attenuation of the X-ray radiation and therefore good vessel contrast. It is therefore generally possible to assess the vessel state effectively. It becomes more problematic when already calcified plaques have formed in the vessel walls. The limited resolution of the CT system and the CT procedure per se mean that the known effect of “calcium blooming” frequently results. This effect causes calcifications to appear much larger than they actually are in the CT image data records. Stenoses produced by the calcified plaques therefore appear much larger than they really are so that the degree of stenosis is overestimated.
To avoid this problem, it is possible to differentiate between calcium and iodine with the aid of dual energy CT examinations, the differentiation having been made up to now on the basis of what is known as the dual energy ratio. To this end the image point values, assigned respectively to identical image point positions in both image data records generated with different X-ray energies, are divided by one another. Image points in the context of embodiments of the present invention refer to the individual voxels and pixels of the image data records. The image point values are therefore the intensity values or the like for the individual voxels and pixels, which are generally determined by means of appropriate reconstruction procedures based on the measured detector values. Generally in X-ray procedures, which include CT procedures, these values are given in the form of Hounsfield values (HU values). Similarly the image point positions in the context of the present invention refer to the voxel and pixel positions. To determine the dual energy ratio, the HU values of the low-energy image are generally divided by the HU values of the high-energy image based on the voxels or pixels. The voxel or pixel in question is then assigned to a material, in this instance either calcium or iodine, based on this dual energy ratio. However problems still arise here due to calcium blooming. If calcium and iodine are directly adjacent to one another, the HU values of the iodine voxels close to the boundary with the bone are falsified by the nearby calcium due to the blooming. This results typically in a rise in the HU values as a function of the X-ray tube voltage, i.e. as a function of the X-ray energy, and therefore also an undesirable change in the dual energy ratio. If the threshold value used to differentiate between calcium and iodine is set precisely at a value between calcium and iodine, if the calcification is extensive enough, the degree of stenosis may be correctly determined. However the remaining voxels in the boundary region with the calcium are falsified, which can among other things cause vessel parts to be eliminated in the image data, in other words to be no longer visible. If however a lower threshold value is used, the remaining image looks better visually but the stenosis is again overestimated, because too high a calcium component remains in the image data.
Another possibility for reducing an overestimate of the degree of stenosis due to calcium blooming is to record CT data as reference image data before administering a contrast agent. This reference image data can then be subtracted from the CT angiography data. Since calcium blooming results in both data records, the effects are canceled out on subtraction and only the lumen without calcification remains. But this procedure has a number of disadvantages. Firstly an additional measurement is required. Secondly there is a significant problem in that a long time interval has to be left between the two measurements due to the waiting time until the contrast agent has spread in the body of the patient. Complex registration (i.e. geometrical matching) of the image data is therefore required to at least reduce motion artifacts, as otherwise the results may be falsified.